1. Field of the Invention
This invention relates to an apparatus for locking a bending mechanism that bends a reflex type wavelength selection element constituting a part of an optical resonator to correct wavefronts in laser beams outputted from the reflex type wavelength selection element.
2. Description of the Related Art
Excimer lasers are employed as light sources in steppers for use in semiconductor device manufacture.
FIG. 6 is a diagram schematically illustrating a whole configuration of an optical resonator 13 used in an excimer laser.
As shown in FIG. 6, a laser chamber 14 of the optical resonator 13 has discharge electrodes 15 which are made up of an anode and a cathode arranged opposite to each other in the direction perpendicular to the paper surface of the drawing. The laser chamber 14 contains laser gas composed of halogen gas, rare gas, buffer gas, etc., which is excited by discharge between the discharge electrodes 15 to generate laser oscillation.
The laser chamber 14 has two laser output ports which are provided with windows 16. Also, slits 19 for restricting the width of the laser beam are provided between the laser chamber 14 and a front mirror 17 and between the laser chamber 14 and a band-narrowing module 18.
The band-narrowing module 18, in this case, comprises a beam expander 20 and a grating 1 serving as an angle dispersing type wavelength selection element. The beam expander 20 is composed of one or more prisms not shown and expands the beam width of the incident laser beam to output it to the grating 1.
That is, in FIG. 6, the optical resonator is configured between the front mirror 17 and the grating 1.
In the construction shown in FIG. 6, a laser beam generated in the laser chamber 14 comes into the band-narrowing module 18 wherein the beam expander 20 expands its beam width. Further, the expanded laser beam L comes into the grating 1 to be diffracted, whereby only a predetermined wavelength component of the laser beam comes out of the grating 1 and is returned in the same direction as the incident laser beam. The laser beam returned by the grating 1, after having its width reduced by the beam expander 20, comes into the laser chamber 14. Namely, the grating 1 serves as a reflex type wavelength selection element that reflects a laser beam and selects a wavelength.
The laser beam amplified through passing inside the laser chamber 14 comes into the front mirror 17, whereby a part thereof is extracted as the output beam and the remainder is again returned into the laser chamber 14 and amplified.
Due to various causes, the laser beam wavefront comes to have divergence (expansion) and curvature inside the optical resonator.
For example, in case that the slit 19 is arranged in the optical resonator 13 as described above, the light having passed through the slit 19 becomes a spherical wave due to the diffraction produced at the slit 19.
Further, the aberration of optical elements arranged in the optical resonator 13 might cause the wavefront to be distorted. For example, a transmission type optical element such as the prism expander 20 used as a band-narrowing element has such properties as follows,    (a) the internal refractive index distribution is not perfectly uniform, and    (b) the polished surfaces of the prism are distorted.            Due to these properties, the wavefront of the laser beam having passed through the above-mentioned type optical element comes to have either convex or concave curvature.        
When the laser beam L whose wavefront has such curvature comes into the flat grating 1, the wavelength selectivity of the grating 1 lowers. Specifically, when the incident wavefront of the laser beam L on the grating has curvature, the laser beam L comes into respective grooves of the grating 1 at different angles, whereby the wavelength selectivity of the grating 1 lowers.
In a conventional art, the above-mentioned problems have been coped with by bending the grating 1 itself according to the curvature of the incident wavefront on the grating so as to coincide with the wavefront of the laser beam coming into the grating.
FIGS. 7A and 7B show the conventional art mentioned above.
As shown in FIGS. 7A and 7B, the grating 1 is provided with supporting members 9 for supporting its both end portions, a holding member 21 for holding its center portion, and a bending mechanism (pushing member 2, spring 3) for moving the center portion of the grating 1 in the pushing and pulling directions through the hold member 21. The bending mechanism can bend the grating 1 in any degree (or can correct the degree of bend).
More particularly, as shown in FIG. 7A, when the wavefront is concave viewed from the traveling direction of the incident laser beam L, the center portion of the grating 1 is moved in the incident direction X1 via the holding member 21, whereby the incident surface of the grating 1 becomes convex. On the other hand, as shown in FIG. 7B, when the wavefront is convex viewed from the traveling direction of the incident laser beam L, the center portion of the grating 1 is moved in the converse direction X2 to the incident direction via the holding member 21, whereby the incident surface of the grating 1 becomes concave.
As the pushing mechanism, a micrometer was used so that the grating 1 precisely is bent in nanometer level.
In such conventional art, since the grating 1 is bent with high precision at nanometer level of measurement, even a very small displacement in bending is not allowed. Accordingly, once laser devices are produced at a factory and the grating has been bent and adjusted, it is necessary not to permit any displacement of the bend from an adjusted position.
However, the configuration of the grating may easily be changed by vibrations and shocks generated during the transportation of the laser devices.
For this reason, there is a problem that the configuration of the grating that has been bent by using a micrometer with high precision cannot be maintained.
Thus, the configuration of the grating does not correspond to the curvature of the incident wavefront on the grating, so that the wavelength selection performance lowers.